Cassidy: Facebook is part of a high-tech posse fighting child pornography

Keyswords such as "10yr," "10yo," "11yr," "11yo," etc. are used by the the Silicon Valley Internet Crimes Against Children (ICAC) Task Force to search a suspect's hard drive for child pornography in San Jose on Thursday, January 10, 2013. (LiPo Ching/Staff)

Emily Vacher's work at Facebook offers a jarring juxtaposition to the Menlo Park company's whimsically decorated cubicles and upbeat, thumbs-up logo. She is the force behind the effort to stop pedophiles from sharing child pornography on the world's biggest social networking site.

Think of it as Facebook CSI.

"What I'm good at," says Vacher, a former FBI agent, "and what I enjoy doing, is bringing these bad guys to justice."

Bearing the reassuring title "trust & safety officer, the Americas," Vacher oversees dozens of Facebook employees who use the latest innovations to spot child porn and scour it from the company's site, or block it from ending up there in the first place. But that's just the start.

It turns out Facebook is a link in a virtuous chain: The company detects child pornography and reports it to the National Center for Missing & Exploited Children, which receives reports from companies and individuals nationwide. The national center then forwards the tip, sometimes adding helpful information of its own, to the proper local police agency.

When things work right, the result can be the arrest and conviction of a bad guy or the rescue of a child being sexually abused. At the very least, a damaging photo is removed from the digital landscape -- or blocked from being uploaded.

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Even that's huge: Like everything on the Internet, child porn can spread at light speed. When photos of child sexual abuse and exploitation are e-mailed or uploaded, the image can go from one person to 20 to hundreds to thousands in a matter of hours. The digital images can live on forever, following young victims through childhood and into adulthood.

"When people view an image again and again, what it does is it re-victimizes the child," says Vacher, who adds that Facebook wants to help other companies use the available tools "to get all child pornography off the Web as best we can."

All along the virtuous chain, investigators are deploying high-tech tools -- from complex algorithms that brand a photo as child porn to computer-packed vans that roll up to suspected child pornographers' homes and serve as instant forensic labs. Within minutes, police investigators armed with search warrants can pour over a seized hard drive, thumb drive, DVD collection or other storage devices and find evidence justifying an arrest -- or determine that the suspect is innocent after all.

And while the battle between high-tech good guys and high-tech child pornographers is something of an arms race -- new technology on the law enforcement side leads to new methods on the bad guy side -- a relatively new tool, called PhotoDNA, has made it advantage good guys for now.

PhotoDNA, which was developed by Microsoft and Dartmouth College, is a tool that assigns an algorithm, something of a digital fingerprint, to child pornography photos. The fingerprint can be used to find copies of the photos (even if they've been edited) posted on the Internet. And as new images are discovered, they are assigned their own fingerprints.

For its part, Facebook has deployed PhotoDNA not only to find child pornography on its site, but to block images from being uploaded. Just as important, PhotoDNA alerts Facebook to the attempted upload, which allows the company to report the attempt to the National Center for Missing & Exploited Children, which begins an initial investigation and contacts local law officers.

Facebook is certainly not the only Internet company taking aggressive steps to stamp out child pornography, but given the company's enormous size (its one billion users upload 300 million photos a day) it seemed like an illuminating case study. PhotoDNA, for instance, is also used by Microsoft, of course, and other companies, which have not publicly announced that they've deployed it, according to the National Center for Missing & Exploited Children.

"It's basically made things easier, so that we can find people who are sharing child porn on the Internet much easier," San Jose Police Sgt. Greg Lombardo says of the technology. "They can just have a computer running all the time, searching."

And when an Internet company gets a hit, it becomes a tip that makes its way to local cops. "We get three or four every single day, seven days a week," says Lombardo, who heads the 11-county Silicon Valley Internet Crimes Against Children Task Force. The number, which reached more than 1,500 last year, is increasing steadily.

While the so-called cybertips are only one way police build cases against child-porn traffickers, they're a valuable tool. The task force doesn't keep a specific count, but there is no doubt that cybertips do lead to arrests and convictions. On Thursday, an 18-member task force team followed a cybertip initially provided by Microsoft to a first-floor apartment in Alum Rock. Inside they found a 22-year-old man with a custom PC bearing digital files containing roughly a dozen photos they identified as child pornography.

The man was arrested on the spot.

Contact Mike Cassidy at mcassidy@mercurynews.com or 408-920-5536. Follow him at Twitter.com/mikecassidy.